The Trump administration is creating a registry for all people who are in the U.S. illegally, and those who don’t self-report could face fines or prosecution.

The move, which could expose unregistered migrants to criminal prosecution, represents a drastic escalation of the administration’s efforts to push millions of immigrants to leave on their own.

But lawyers and immigrant advocates warned that the administration’s plan was on shaky legal ground and that some immigrants may be afraid to comply.